1. Technical Field
The present invention relates to a boiling water nuclear plant and a method of reducing dose in a turbine system, and more particularly, to a boiling water nuclear plant and a method of reducing dose in a turbine system which accepts, as power energy, steam generated in a boiling water reactor.
2. Background Art
In a boiling water nuclear plant, radioactive nitrogen (N-16) is generated from the reaction of oxygen (O-16) in reactor water with neutrons. This N-16 has a half-life of 7.1 seconds, emitting high-energy gamma rays (6.129 MeV). Among the generated N-16, the N-16 in chemical form of high-volatile ammonia (NH3) or nitrogen monoxide (NO) does not remain in the reactor water but is volatilized into steam introduced to a turbine, causing an increase in dose of a turbine system.
Recently, in the boiling water nuclear plants, hydrogen injection is executed in order to prevent stress corrosion cracking in structural material of a reactor pressure vessel and reactor internals by decreasing the amount of dissolved oxygen in the reactor water in the reactor pressure vessel. However, when the amount of injection of the hydrogen is increased, a radiation dose rate in the turbine system tends to rapidly increase after reaching a certain amount of the injection of hydrogen. This is because some N-16 dissolved in the reactor water in a low-volatile chemical form such as nitrate ions during a normal operation are reduced by the hydrogen injection, turn mainly into NH3 which is a high-volatile chemical form, and move with the main steam. Because of the increase in the radiation dose rate, an upper limit is set for the amount of hydrogen to be injected.
As a conventional technology of reducing the amount of N-16 introduced into the turbine system, moving with the main steam, a method has been proposed where the amount of N-16 volatilized into the steam is decreased by adding an agent for reacting with nitrogen compounds to form nonvolatile nitrogen compounds to the reactor water (see Japanese Patent Laid-open No. 2009-109318 for an example).
In addition, a method has been proposed in which N-16 in a high-volatile ammonia form is oxidize into a low-volatile nitrogen oxide by function of a photocatalyst layer containing an ammonia-adsorbing layer provided to a location that Cherenkov light generated in a core reaches (see Japanese Patent Laid-open No. 2009-281893 for an example).